Enthalpy

Nice college! I'd have preferred that to experiments with falling balls... And sure, a good source makes it easier. ---------------------------------------- RKBrooks, if you liked the Musée des Arts et Métiers, you should try the Deutsches Museum in Munich. Much bigger, better explained, with nice historic pieces as well (Siemens' electric railway engine, first induced fission, things from Lilienthal and Wright...) and more actual technology. And the Technorama as well, in Winterthur.

The safe Li-FePO4 brings 371kJ/kg. A flywheel of good steel (400m/s toroid) stores 80kJ/kg, of graphite-epoxy (800m/s) 320kJ/kg - in ideal cases, and exploiting the energy down to zero speed.

Oops. I wanted nearly parallel light converging to a small image of the Sun, and this is not compatible. A small image implies a strong convergence. Thermo's second law tells it, but knowing geometrical optics would have been even better... The good news is that a small image accepts a long path in the telescope. Just one example here, where rotating mirrors orient the thrust, and this movement adds no aberration: Stacking many mirrors in a launcher's fairing constraints such designs. And with several engines and a craft, the control must avoid collisions, as the jet impact must be hot and corrosive.

Much Sunlight power is needed, so most uses will have several concentrators as big as the launcher's fairing permits. They can consist of metallized graphite honeycomb, like usual satellite antennas. One chamber per concentrator gives redundancy and eases the orientation and the ground tests. D=4.4m provides near Earth 16.6kW if 80% are used, enough for 195mg/s hydrogen to push 2.4N. The chamber's light inlet must be small because it's hot: at D=38mm, blackbody's radiation loses 19% of the incoming power. Hence the chamber must be close enough to the primary mirror (here 0.92 diameter) that the Sun's image fits through the inlet. The small secondary mirror makes light more parallel. At D=0.2m, the temperature can be kept bearable by the reflective surface. The nozzle must be oriented, using the secondary mirror and possibly more. A different telescope formula can bring advantages, but it must keep light's path short. Marc Schaefer, aka Enthalpy

NBR is commonly used as a seal joint material, and provided someone wants it to biodegrade, a PHB filler doesn't look that promising: http://en.wikipedia.org/wiki/Polyhydroxybutyrate - High permeability - Poor resistance to acids and bases (but might match hydraulic oil) - I would not use a rigid filler in an elastomer! Why should a filler be better than a copolymer?

Dear visionary inventors, megalomaniac engineers and audacious explorers, Chemical rocket engines aren't up to our desire to hop in the Solar system: go quickly to Mars, deviate Earth-threatening objects, and so many more missions. We need a higher ejection speed to save propellant mass, but this takes more energy than chemical reactions bring. One possibility is to tap Sunlight instead of transporting the energy. I suggest - as others did - to directly heat the propellant with Sunlight. Converting first to electricity would enable even higher ejection speeds, but direct heating is energy-efficient, so for a long weak thrust, the collector area is feasible - smaller and of cheaper materials than Solar cells or even a heat sink. Material sublimation limits thermal designs to <3000K, so only hydrogen improves the ejection speed over a combustion. My plan is to also dissociate a part of this hydrogen to increase the ejection speed further. -------------- The heater that catches concentrated Sunlight and transfers heat to hydrogen is of tungsten alloy. At 2800K =2527°C, sublimation thins it by 45µm in 14 days, from Plansee's doc. 30mbar in the heating chamber let 23% of injected H2 split to atomic H* for performance. Expansion to 1Pa in the nozzle leaves 15.6MJ/kg from 92.8MJ/kg in the chamber, for isp = 1267s = 12.4km/s. Mean 2800K is already a lot, as sublimation is very sensitive to it. The nozzle can't grow much because the molecules' mean free path is already ~10mm. A lower chamber pressure dissociates more hydrogen and improves the ejection speed, but needs much more heating power, as the nozzle gets inefficient - recombining hydrogen is a hard task. But more pressure brings during some flight sequences more thrust traded against ejection speed: for instance, 0.8bar and 2093K from the same Sunlight concentrator and more hydrogen throughput push *2.1 times stronger with isp=800s. More to come. A former version, partially inaccurate, began there http://saposjoint.net/Forum/viewtopic.php?f=66&t=2164 and an actual version has begun there http://saposjoint.net/Forum/viewtopic.php?f=66&t=2164&p=42029#p42029 which I plan to describe more concisely in the coming few days on ScienceForums, so stay tuned! Marc Schaefer, aka Enthalpy

The Milky Way is bigger than that! http://en.wikipedia.org/wiki/Milky_Way we're about 20,000 light-years from the disk's edge. At 0.99*c, the contraction is 0.14, or 2,800 years for the travellers. 20,000 years observed from Earth.

Fun: the historical experiment could easily be reproduced at a school, university... Taking all legs 100m long this time, and rotating the mirror at 100Hz (6000 rpm) with a small DC motor, the deviation is 42mm, wow. A laser pointer diverges too much over 300m, but with an additional lens? Or maybe a halogen lamp, a lens and a far enough collimator? The rotatig mirror should better have many polished facets to increase the received light intensity.

If you ask whether a controller is necessary to rotate a brushless motor, yes, building the motor and the controller is to difficult for the moment. Less difficult (but already difficult enough!), I'd suggest the vacuum aerostat http://saposjoint.net/Forum/viewtopic.php?f=66&t=2520 accessible to a model airplane hobbyist. Or the Pelton-Schaefer pump: http://saposjoint.net/Forum/viewtopic.php?f=66&t=2272&start=10#p27829 http://saposjoint.net/Forum/viewtopic.php?f=66&t=2272&start=50#p33837 needs access to a milling machine. Even more fun, reproduce the historic Fizeau-Foucault experiment http://www.scienceforums.net/topic/16198-rpm-of-fizeau-foucault-apparatus/#entry752353 looks easy. It needs the proper light source and little more.

If it's really carbon (or graphite; some suppliers call graphite the purer carbon, others make no difference) it will not react to a flame. That's why graphite serves to build ovens. For kites, model airplanes... carbon rods are made of carbon fibers in a polymer matrix. Transforming them into a carbon-carbon composite would be a too big effort. As the polymer is graphitized (under controlled atmosphere) it must be replenished with some tar-like material, normally done under vacuum. Hydrocarbon gases are also used. Alumina powder is available commercially to make parts by mixing it with water, much like plaster. Make a crucible, try it at the proper temperature. Maybe you find a concave stone in a river? Not recessarily the perfect shape, and it may break at heat. An induction furnace must be feasible, if you have solid knowledge of electromagnetism, but that's a significant effort just to replace a blowtorch. You might take a used induction coker and redesign the magnetic parts.

I expect them to have observed a deflection distance to know the angle, and a long path allows to measure angles more precisely. Then, two options: - Observe the spot at the same experiment site as the rotating mirror. Less sensitive, but limits people's travels. Probably what they did. - Have the rotating mirror at one site, the fixed one and the observed spot at the other. More sensitive, but complicates the interactions between the crews at the rotating mirror and the observed spot. The one I had imagined. As they chose 35km, it was probably the first option, yes. The second would have allowed a distance like 1km.

Hydrogen bonds can have varied strength. Not only because of proximity effects in the molecule (compare an carboxylic acid with an alcohol), but also because in a solid, not all bonds may find the optimum angle to the neighbour molecules - or even, some bonds may be frustrated because of geometry. There is also a more general reason. Dissolution, or mixing, (or chemical reactions) also happens when it is energetically neutral or slightly defavourable. For instance glycerine or erythritol mix or dissolve well with water but the mix is very cold to the hand. Imagine one single sugar piece dropped in the Ocean. The sucrose molecules dissolve, drift away, and have no chance to deposit again on the sugar piece. The sugar piece dissolves completely, even if the reaction was defavourable. To get an equilibrium, the amount of water must be smaller, so that the concentration of dissolved sugar reaches an equilibrium, where as many molecules per second dissolve and crystallize. This "equilibrium concentration" (or "saturation concentration" for a dissolution) depends on the temperature and, for a gas, on the pressure. A statistical computation of dissolved molecules gluing or not to the solid would be extremely complicated, but fortunately, thermodynamics can relate the equilibrium concentration to simple values like the heat of dissolution. In thermodynamics, what drives reactions and equilibria is not the heat of reaction but the entropy, which also depends on concentrations (though a big heat of reaction may overwhelm other contributions in entropy and decide alone). By the way, the conservation of energy already tells that heat alone can't decide what direction a reaction will take, because if chemical energy has decreased, energy has increased somewhere else. The view of thermodynamics is instead that energy tends to dilute naturally, and entropy is a measure of how equally distributed heat is. Take a meteoroid as an example. Drop it from 100,000km height: it and Earth lose gravitational energy which heats the meteoroid, the air and Earth. No energy has been lost, but it was previously concentrated in one dimension (the height of the object) and is diluted into heat after the impact, where each atom has gained kinetic energy, so heat is energy spread over >1023 dimensions. Entropy tells the falling rock story happens, while a hot rock won't spontaneously climb - energy alone would not tell it. Because heat is very dilute energy, conversions into heat occur easily, while the conversion of heat into work (engines), electricity, chemical energy... is difficult and demands engineering and machines.

If the air has pressure PA and the gas in the cylindre PC (at some time since P will vary), then the piston displacement making dV volume reduction: - gets PA*dV work from the atmosphere - gives PC*dV work to the gas in the cylinder - becomes (PC-PA) *dV work from the force on the piston. So a force is necessary when PC differs from PA - usually the case, for instance because PC varies upon compression.

When a complex number represents the amplitude and phase of a sine wave, it has a strong reality and is concrete to many people, electronicians for instance.

If they observed a deflection of 60m over 35km between the rotating mirror and the observers, then 0.1° was enough, needing only 1.7s per turn. Impressive sensitivity.

The size of a fundamental particle is a tricky idea. It depends essentially on the way you want to measure it. If using a slow atom to measure the size of an electron on other atom's outer shell, the answer is "the electron has the size of the atom". But if using a smaller tool to measure the electron's size, the answer gets smaller as well. Use a GeV electron to measure an other electron, for instance the one that was as big as the atom, then the measured electron will give the result expected from a particle that is at one small location - but the chances of being in one particular location decreases with the observed volume. Up to now, all particles used as a tool, even the "smallest" ones (which often means the highest energetic ones) have seen the electron in a location small enough to look like a point. I that sense, it's a point particle. But: - Some day an even smaller tool may find a size to the electron - If the tool is big, so can be the electron The difference is rather with a composite particle like a nucleus or a proton. Small tools like high-energy protons "feel" the nucleus as a collection of smaller objects spread over a definite volume which keeps its size as the "tool" gets finer. It's essentially the same story with the photon. A photon can be big or even so huge that one can't imagine it. Take a photon emitted by a star 1 light-year away, in a cone of 1 steradian and lasting 1ps: its volume is 1028m3. But through a telescope, it can be detected by one pixel of a camera, say 5µm*5µm, and then we may decided that the photon was on that area. The chances of catching it were just small. So the size is not a fixed property of an elementary particle. If a particle can be as local as our tools distinguish, we may call it "elementary". And the idea of "particle", especially the photon in the history, has much to do with this capacity to act locally if the "tool" (here the camera pixel) needs it.

The kind of fatty acids eaten seems to influence acne: saturated ones seem to be bad here as well. Then, vegetals would be an imperfect targeting, as palm oil is saturated while some fish has excellent fats. Antibiotics work very well against acne, but can't be taken permanently.

Electric motors start quickly, an advantage for an ambulance hexrotor with fuel cells. Here some figures. Three rotors at each side take length, so the body shall accommodate two beds, plus four medics and a pilot. Mass estimate: 2* 110kg - - - Patients, dress, beds 1* 100kg - - - Medical apparatus and supplies 5* 90kg - - - Four medics, one pilot, dress, seats 250kg - - - Cabin 100kg - - - Truss 50kg - - - Armour 120kg - - - Parachute 100kg - - - Infrared vision, terrain radar 6* 125kg - - - Rotors, stators, motors, electronics 6* 100kg - - - Fuel cells 200kg - - - 100kg hydrogen in tank ========== 2850kg - - - Take-off mass Six fuel cells provide more power to lift more mass with still D=3.8m rotors. The degraded mode with 4 rotors active accelerates 4*326kg/s air to 23.5m/s with 60% efficiency to lift 30.7kN or 1.1g. The degraded mode with 5 fuel cells lifts 1.1g at no cell overload. Stationary normal flight accelerates 6*254kg/s air to 18.3m/s, consuming 425kW. If flight consumes 550kW as a mean, 60% efficient fuel cells use 100kg hydrogen in 4.3h. A sketch is coming. Marc Schaefer, aka Enthalpy

CS: Counter-strike? Corticosteroids? Computer Science? Congregation of the Missionaries of St. Charles? ------------------------------------------------- No idea if this fits your desire, but I strongly wish one software, and you might be sensitive to it in the Philippines... The Diccionario de la Real Academia Española (DRAE) is recognized as the best Spanish dictionary. One software edition exists (simply "Edición en Cd-rom", vigésima segunda edición, by Espasa), but it is, how to say, less than perfect... Written in Java (first mistake), it seems to load the full database in Ram (second mistake) before the software starts, which takes nearly one minute, making the software nearly unusable. So I'd like a good new software that exploits offline the existing DRAE database, as the existing software does, but better. As models, the "Dictionnaire Hachette-Oxford" for English <-> French has a database of similar size but starts instantly; the "Deutsches Universal Wörterbuch" von Duden does as well, and many more. I suppose it suffices to program cleanly and sensibly, using good standard database techniques. There is a market for it (if customers realize it's not the bad old software). Maybe the Academia is interested (they own the database anyway), or rather a Latin American editor, as the economic situation suggests. ------------------------------------------------- One other wish: I'd like a software that finds the personnal settings I make on Windows and writes them as a .reg file for further use. Especially the ones for Windows Explorer and the "search file" function: I waste half an hour after each Windows installation, after which I haven't still changed all the settings I need. I've found no such software: - Run it late, when the settings are already made - Keep the settings in a file for reuse. I don't reinstall or upgrade a system here, it's for fresh install. - Run between different editions of Windows - including old editions knowing one single user. ------------------------------------------------- Other project ideas: http://www.scienceforums.net/topic/69084-suggest-a-topic-for-master-thesis/ http://www.scienceforums.net/topic/68796-need-some-ideas-for-a-project/ http://www.scienceforums.net/topic/68996-compression-on-cuda/ especially the file digest is simple, to find files renamed or moved, in a file synchronizer http://www.scienceforums.net/topic/68781-i-need-suggestion-regarding-my-master-thesis-topic/ more ambitious for one person in 8 months Fingers crossed, because these are software pieces I'd like to have! Marc Schaefer, aka Enthalpy

To understand CO2 in the atmosphere, the best way is to follow the carbon. Carbon can be in the soil, put there over hundreds of millions of years in the form of coal, petrol, gas, peat... Or on the soil as organic matter, in forests, crop... Or in the Ocean as biomass, dissolved CO2, methane hydrates. Or in the atmosphere as CO2. If you think at the transformations between the molecules containing carbon, and their location, you understand more. For instance, that a forest stable in height and extension does not transform CO2 into O2 - it only stores carbon as organic material, this carbon being away from the atmosphere. From the excess CO2 we inject by burning fossil fuels, the Ocean takes a part, the rest is in the air to stay. The proportion of atmospheric CO2 has already risen a lot, the amount fits the quantity of fossil fuels Mankind has burnt, and the epoch of CO2 rise fits the consumption of fossil fuels. So independent thinkers don't need to believe one group or an other, nor study twisted arguments. All the data is available. Compare it, and conclude that Mankind has increased the atmospheric CO2 by burning fossil fuels.

I had in mind that big telescopes for visible wavelengths were coated with gold, but apparently it's aluminium or silver, protected by Si3N4. My bad. Reflectance versus wavelength, from Wiki: even metals looking very white, like platinum, reflect less.

A matter of distance, that's my opinion too. Direct sight to >10km altitude goes farther than 50km: it would be 357km on a perfectly spherical Earth - a big attenuation and delay for sound. Such silent thunderstorms are not uncommon in Europe. I doubt wind could have an effect on it. The effect of wind on sound is intriguing, because the simple change of path length would have no noticeable effect: even 30m/s wind would make the path just 1/10th longer for sound, and let it lose 1dB only due to open-space attenuation. The best explanation I got is that near the ground, wind has a strong gradient due to viscosity; this gradient also carries sound at a speed that changes with height; upwind, the propagation speed gradient lets the wave bend upwards and vanish to us, while downwind, the gradient bends the wave downwards and guides it like fiber optics.

What values do you want for sin and cos? Rational as in your last post, or algebraic like in the first? By the way, cos(π/3)=0.5 is rational despite 3 not being a power of 2. Could you detail what you expect from the angle and from the sine?

The first message's claim is doubtful. We know that a photon has existed when it is destroyed, and at no other time - except maybe in some very subtle recent experiments. So what is meant by "pop out then return" and by "duration of light"?

Already the crust breaks of flows in response to limited pressure differences This tells why mountains are only 9km high, as compared with 6400km Earth radius. The core is hot enough for iron and nickel to be liquid. But currently fashionable theories want the inner core to be a solid, as pressure suffices there to create an iron-nickel crystal with a higher fusion temperature. In any case, the hydrostatic pressure exceeds the materials' yield strength so they flow - much more so because heat reduces the yield strength, and because over geological time, "flow" is more a very slow creep, against which hot materials have very little resistance. So what shall "solid" mean? It's "measured" in the laboratory over a short time and essentially no shear stress. As well, some shear resistance over the duration of earthquakes waves would fit observation well - this is compatible with creep over geological times.